DE1667569C3 - Process for the production of Maddrell's salt - Google Patents

Description

35

The present invention relates to a process for the production of Maddrell's salt by heating of sodium dihydrogen phosphate to temperatures of 450 "C.

It is known to produce Maddrell's salt by dehydrating sodium dihydrogen phosphate. However, depending on the reaction conditions, more or less large amounts are formed during dehydration Shares of water-soluble phosphates, such as acid sodium pyrophosphate, sodium trimetaphosphate or long-chain, soluble polyphosphates.

According to F. Thilo and R. Ratz (Journal for inorganic and general chemistry, 258 [1949], p. 53), Maddrell's salt is produced by heating NaH ₂ PO ₁ to 35OT for days and washing it out with water despite the long tempering times Soluble phosphates that are still present or formed are freed.

DEC Cοrbridge and FRTromans (Analytical Chemistry, 30 [1958], p. 1101 to IUO) also produce Maddrell's salt by tempering NaH ₂ PO ₄ at 380 ° C. for days and then washing the tempered product.

In a technical process for the production of Maddrell's salt (US Pat. No. 2,356,799), NaH ₂ PO ₄ with up to 5% water is pressed into tablets and ^{heated in a rotary kiln at 300 to 460 °} C. in a stream of hot air. The products produced here are made only 95 ° / ₀ from Maddrell salt (see. Journal of American Chemical Society, 81 [1959], p 79) and contain varying amounts of soluble phosphates which practically be getting more than 1 weight percent.

Brauer mentioned in the »Handbook of the preparative inorganic chemistry «, published by Ferdinand Enke Verlag, Stuttgart, that the production of Maddrellschem Salt occasionally fails completely for reasons that have not yet been explained.

In many cases, especially when Maddrell's salt is used as a toothbrush body it is desirable to keep it as free from water-soluble phosphates as possible. Aimed for are products with less than 1 percent by weight of soluble phosphates, as the latter is known have strong complexing properties and therefore attack tooth enamel.

It has therefore already been proposed for the production of water-insoluble sodium polyphosphate, sodium dihydrogen phosphate, after a short warm-up time to around 150 C, with a constant Heating rate between 2 and 20 C / min to about 450 C, then heat the product to keep about a third of the time required for heating at this temperature and the temperature that forms To remove water vapor continuously from the oven.

It turned out, however, that this way of working by no means always and in a more targeted and High-proof Maddrell's salt was always produced in a reproducible manner.

It has surprisingly been found that the conversion of NaH ₂ PO ₄ into Maddrell's salt is dependent on the surface of the starting product and only in the temperature range from 200 to 450 ° C. on the water vapor partial pressure in the reaction mixture.

Specifically, the present invention consists in that, for the production of Maddrell's salt by heating NaH ₂ PO ₄ to temperatures of 450 ° C., the starting product is NaH _B PO ₄ with a surface area of at least 0.1 m ² / g, measured according to BET used and during the heating in the temperature range from 200 to 450 ° C, the water vapor partial pressure within the reaction mixture is kept between 50 and 450 Torr.

The desired water vapor partial pressure within the critical temperature range is thereby by regulating the heating rate and / or by simultaneously passing over less water vapor Gases, in particular from air, set on the reaction material.

The table below shows that the conversion of NaH ₂ PO ₄ into Maddrell's salt is only dependent on water vapor in the temperature range from 200 to 450C.

Table I.

Heating of NaH ₂ PO ₄ at a heating rate of 15 ^C C / min

Steam Steam Weight percent partial pressure, partial pressure, Maddrell's salt measured in Torr measured in Torr Difference to 100 in temperature in temperature corresponds to quantity Section Section of soluble 20 to 200 ⁰ C 200 to 450 ° C Phosphates 280 280 99.4 470 300 98.4 300 470 62.0

It is advisable to regulate the rate of increase of the reaction material to cause the reaction material through a H-: izzone with accordingly set temperature profile is moved. Heating speeds of less than 50 C / min, preferably less than 30 C / min, are advantageous here.

NaH ₂ PO, with a surface area of at least 0.1 m ² / g, is preferably obtained by vacuum drying NaH ₂ PO ₄ -2H ₂ O or by spray drying a NaH ₂ PO ₄ solution, in particular by drying NaH ₂ PO ₁ - 2H ₂ O in vacuo at a pressure less than 80 torr. Advantageous has been found that reaction mixture 200 välzen one additional switch to 450 ^c C to accelerate the diffusion of water vapor during the normal horizontal movement in the Teniperatur /.

If the water vapor partial pressure prevailing in the reaction mixture falls below or exceeds the stated limits, _{a considerable proportion of sodium trimetaphosphate is formed from the NaH 2} PO _{4 used} when heated to 450 ^° C. in addition to Maddrell's salt.

In the following tabulated examples, NaH ₂ PO _{4 was} heated to 450 ° C. at heating rates of 15, 8 and 5 ° C./min under various steam partial pressures and the proportion of Maddrell's salt was determined analytically in the end product.

The individual values are shown in FIG. 1 plotted graphically.

Table 11

Steam Weight percent partial print Maddrille's salt Heating Difference to 100 speed 3 corresponds to quantity
of soluble 8th Phosphates 15th 34 76.5 15th 80 78.7 15th 120 80.0 15th 280 98.5 15th 365 99.0 15th 405 99.4 15th 410 99.3 15th 470 97.6 15th 505 95.2 15th 20th 45.0 15th 60 41.0 8th 150 51.0 8th 320 98.5 8th 410 98.7 8th 465 99.1 8th 485 99.0 8th 505 93.0 8th 22nd 89.0 8th 32 19.0 5 40 45.0 5 63 75.0 5 116 94.1 5 196 97.9 5 368 98.0 5 418 98.5 5 490 99.3 5 99.8 5 93.0

The surface area of the NaH ₂ PO _{4 used} was 0.25 m * / g measured according to BET in all cases.

The production of Maddrell's salt is expediently carried out in an apparatus as described in F i g. 2 is shown schematically.

In a 2 m long furnace, a temperature profile of the type shown is set by applying appropriate heating windings (not shown). Boats made of VA sheet metal filled _{with NaH 2} PO _{1 are} slowly pulled through the oven at constant speed. Depending on the speed of the boat, air is then sucked out of the furnace at I so that a water vapor partial pressure can be established in the reaction mixture which is within the range given above.

example 1

500 g of NaH ₂ PO ₄ , filled into 10 boats made of ao VA steel sheet, were pulled through an oven according to FIG. 2 at a speed of 5.5 cm / min, corresponding to a temperature increase of 12.5 ″ the amount of air sucked off in the temperature zone 200 to * 5 450 ^° C. 240 l / h The average water vapor partial pressure of 105 Torr prevailed.

The final product (425 g) consisted of 99.4% Maddrell's salt and was easily crushable in the form Easy to take cake out of the boat. The soluble fraction was 0.6%.

Example 2

500 g NaH ₂ PO _4, filled into 10 boat made of VA-steel plate were at a rate of 7 cm / min, corresponding to a temperature increase of 16 ^u C / min, through a furnace in accordance with F i g. 2 pulled through. The suction air was 360 l / h, the water vapor partial pressure 90 Torr. The final product (425 g) consisted of 99.1% Maddrell's salt and was easy to crush. The soluble fraction was 0.9%.

If the speed of the boat was increased, the amount of exhaust air had to be disproportionately high be increased, since at high heating rates the diffusion of the water vapor from the Sample could no longer be done quickly enough.

Example 3

500 g of NaH ₂ PO ₄ were, as described in the previous examples, at a constant speed of 8 cm / min, which corresponds to a temperature increase of 18.5 ° C / min, by a

Furnace according to FIG. 2 drawn. The amount of air extracted was 440 l / h, the water vapor partial pressure was 85 torr. The final product (425 g) was 96% from Maddrell's salt and 4% from soluble parts.

If the amount of exhaust air was increased even further, the heating rate could also be reduced of 18.5 ° C / min, products with a minimum Maddrell salt content of 99% are obtained.

. For example 1 4

500 g of NaH ₂ PO ₄ were, as described in the previous examples, at a speed of 8 cm / min, corresponding to 18.5 ° C / min, temperature

temperature increase, pulled through the furnace. The amount of suction air was 800 l / h, the water vapor partial pressure 50 torr.

The end product consisted of 99.2% Maddrell's salt, the soluble portion was 0.8%.

If too little or no air at all is sucked off, a higher, more favorable, one appears in the reaction mixture Was able to form area exceeding water vapor partial pressure, so also formed in the in FIG. 2 shown experimental setup trimetaphosphate.

Example 5

450 g of NaH ₂ PO ₄ were poured into 9 boats made of VA steel sheet and pulled through the furnace at 3.6 cm / min, corresponding to 8.3 ° C. / min, without drawing off air. In the first boat there was still practically pure Maddrell's salt (99% Maddrell salt and 1% sodium trimetaphosphate), in the second boat the end product was strongly sintered and contained only 77% Maddrell's salt and 23% sodium trimetaphosphate. In the third and the following 6 boats, the end product was very strongly sintered and melted together and contained only 50% Maddrell's salt and less.

The reverse case, i.e. the heating of NaH ₄ PO ₄ under an H ₂ O partial pressure that is too low, was also used in the test arrangement F i g. 2 carried out.

Example 6
50 g of NaH ₂ PO ₄ were poured into a boat made of VA steel sheet and pulled through an oven at a speed of 1.8 cm / min, corresponding to a temperature increase of 4.1 ° C. / min. The amount of suction air was 2600 l / h, the water vapor partial pressure 2 Torr. In the final 75% Maddrell salt and 25 ° / ₀ water content were included.

It was also found, as already mentioned, that in particular _{high-surface samples of NaH 2} PO ₄ , such as those obtained by vacuum drying or spray drying, were particularly suitable for conversion into Maddrell's salt.

Example 7

«5

NaH ₂ PO ₄ · 2H ₂ O, which is obtained by reaction of stoichiometric amounts of phosphoric acid (85 percent by weight) and sodium hydroxide solution (50 percent by weight) and subsequent crystallization, filtration and

»O washing with alcohol had been carried out, was freed from the water of crystallization in a vacuum drying cabinet at 10, 20, 40, 60, 80 and 100 Torr and 90 ^{° C.} 50 g each of the products obtained in this way were, as described in the preceding examples, in one

»5 VA steel sheet boats with one speed of 4.6 cm / min, corresponding to a temperature increase of 10.6 ° C / min, with a suctioned Air flow rate of 50 l / h through an oven according to FIG. 2 pulled through.

The partial pressure of water vapor was 150 torr.

The values obtained are summarized in the table below.

Drying the
NaH ₁ PO, -2 H ₁ O
at surface
of the received
NaH ₁ PO ₄ 1 tone 0.32 m * / g 10 torr 0.20 m '/ g 20 torr 0.15 m * / g 40 torr 0.14 m * / g 60 torr 0.16 m * / g 80 torr immeasurable
«0.1 mVg 100 ton immeasurable
«0, lm ^! /G spray
dries 0.25 m ^s / g

Grain distribution of NaH ₁ PO ₄

From the NaH ₂ PO ₄

received
Maddrell's salt in percent by weight

8% <40μ <7% <60μ <11% <ΙΟΟμ
100μ <24% <200μ <50%

16% <40μ <6% <60μ <8% <ΙΟΟμ
ΙΟΟμ <21% <200μ <49%

4% <40μ <8% <60μ <8% <ΙΟΟμ
ΙΟΟμ <21% <200μ <59%

2% <40μ <6% <60μ <20% <ΙΟΟμ
ΙΟΟμ <28% <200μ <44%

% <40μ <6% <60μ <29% <ΙΟΟμ
ΙΟΟμ <32% <200μ <32%

2% <40μ <4% <60μ <22% <ΙΟΟμ
ΙΟΟμ <26% <200μ <44%

1% <40μ <8% <60μ <9% <ΙΟΟμ
ΙΟΟμ <22% <200μ <60%

46% <40μ <15% <60μ <16% <ΙΟΟμ
100μ <19% <200μ <4%

99.7

99.7

99.4

98.7

99.4

94.7

46

99.1

For this purpose 2 sheets of drawings

Claims (4)

Patent claims: 1. A process for the preparation of Maddrcll's salt by heating sodium dihydrogen phosphate to temperatures of 450 C, characterized in that a sodium dihydrogen phosphate with a surface area of at least 0.1 m ² / g, measured according to BET, is used as the starting material and during the heating in the temperature range from 200 to 450 ° C the water vapor partial pressure within the reaction mixture is set between 50 and 450 Torr by regulating the heating rate and / or by simultaneously passing gases with little water vapor, in particular air, over the reaction mixture. 2. The method according to claim 1, characterized in that the regulation of the heating rate of the reaction material by means of moving the same caused by a heating zone with a correspondingly set temperature profile will. 3. The method according to claim 1 or 2, characterized in that the heating with heating speeds less than 50 "C / min, preferably less than 30" C / min. 4. The method according to any one of claims 2 to 3, characterized in that the reaction material is circulated during heating in the temperature range from 200 to 450 C.